Our invention relates generally to support structures and, more particularly, to a novel and useful heat pipe assembly for use with support structures or foundations in permafrost areas or in any areas having active ground layers subject to a severe annual freeze-thaw cycle.
Permafrost is material which is largely frozen permanently. It is usually a mixture of soil, rock and ice although it can be anything from solid rock to muddy ice. In the arctic regions, permafrost may extend from a few feet to hundreds of feet below the surface. The permafrost is separated from the surface by an upper soil layer commonly called the tundra which supports a dense growth of surface vegetation. The tundra is subject to a seasonal freeze-thaw cycle and serves as insulation to limit permafrost thaw in the summer. The thaw in the summer, however, can create an unstable condition for structures constructed in the arctic regions. This is, of course, more so in wet, ice-rich, permafrost areas than in dry, stable, permafrost areas of well drained soil or rock.
There are severe problems associated with support and stabilization of structures in the arctic regions where permafrost is prevalent. Alaskan railroads, for example, require the expenditure of thousands of dollars each year to repair soil slippages and track roughness resulting from the annual freeze-thaw cycle and disturbances of the ground cover by the intrusion of man and his machines. When the tundra is broken or removed, the permafrost loses its insulation and begins to melt and erode. Thus, tracks left by a tractor or caterpillar train can become a deep ditch and alter the surface drainage pattern over a wide area.
In cities and regions which overlay permafrost areas, a gravel insulating technique is generally used in construction over such areas. A raised gravel pad, for example, is ordinarily employed to provide a suitable support or work area on permafrost. Foundation structures embedded in permafrost are also commonly surrounded completely by a layer of insulating gravel. In areas of ice-rich permafrost and/or during a strong summer thaw, however, even the use of a relatively thick insulating gravel layer is inadequate to prevent some subsidence and possibly accompanying damage of the supported structure or apparatus. On the other hand, instead of subsiding, support posts or poles for arctic overhead communications and power lines have presented a particular problem with "pole jacking" wherein the annual seasonal uplift due to frost heave can actually lift the poles and their anchors completely out of the ground. The pole jacking problem has plagued all of the utility companies throughout vast areas of the arctic and subarctic regions.
The patent application Ser. No. 174,687 of Elmer D. Waters on Permafrost Structural Support With Heat Pipe Stabilization filed Aug. 25, 1971, now U.S. Pat. No. 3,788,389 discloses and claims a cooperative combination of a support structure and heat pipe element installed in generally frozen soil. The heat pipe element is of a suitably complementary configuration and/or disposition with respect to the support structure to provide appropriate stabilization of the surrounding frozen soil. In one embodiment, the heat pipe element is disposed externally of the support structure and, in another embodiment, it is disposed internally of (and integrally combined with) such structure. The external embodiment further includes one version employing a linear (straight) heat pipe element and another version employing an angular (helical) element. In both versions of the external embodiment, an overlapping joint can be provided to join an aboveground radiator section of the heat pipe element to an underground embedded section thereof. This permits the upper radiator section to be readily separated and detached from the lower embedded section at the joint.
The heat pipe element just described above and various other natural convection heat transfer devices used for stabilization of permafrost foundation areas present a problem when complete device replacement is necessary, even if the device is not part of the support structure. Replacement usually requires the digging of a new hole and this may be difficult to accomplish in remote and rugged regions where a drilling rig cannot be easily transported and/or used. In addition, if the particular installation first requires removal of the old device, this may be especially difficult since it is likely to be in frozen ground with the supported structure or associated support structure securely attached to the old device. Of course, essentially the same difficulties are encountered with support structures already installed in areas where stabilization is subsequently found to be necessary.